Selectively targeting solid tumors in vivo is a highly desirable but so far elusive goal for cancer therapy (Dvorak, H. F., et al., (1991) Cancer Cells 3, 77-85; Auerbach, R. (1991) Int J Radiat Biol 60, 1-10; Burrows, F. J. and Thorpe, P. E. (1994) Pharmacol Ther 64, 155-74; Schnitzer, J. E. (1998) N Engl J Med 339, 472-4). Current therapies for solid tumors lack sufficient tumor-specific targeting to avoid systemic side effects (Burrows, F. J. and Thorpe, P. E. (1994) Pharmacol Ther 64, 155-74; Schnitzer, J. E. (1998) N Engl J Med 339, 472-4; Schnitzer, J. E. (2001) Adv Drug Deliv Rev 49, 265-80). Pharmaceuticals conjugated to tumor-cell specific antibodies may show excellent specific activity in vitro; however, when injected intravenously as “magic bullets” to target neoplastic cells inside the tumor tissue they encounter significant barriers in vivo that limit accessibility and reduce bioavailability and bioefficacy (Dvorak, H. F., et al., (1991) Cancer Cells 3, 77-85).